‘Meta-Atoms’ Could Advance Optical Devices

Photonics.comApr 2013
ROLLA, Mo., April 29, 2013 — Specially designed “meta-atoms” capable of stretching lightwaves and accelerating them to a speed faster than light could advance optical devices, a US group has theorized.

The research is the latest in a series of recent findings related to how light and matter interact at the atomic scale. Missouri University of Science and Technology researchers are the first to demonstrate that the material — a meta-atom of gold and silicon oxide — can funnel light through ultrasmall channels at a speed approaching infinity.

The meta-atoms could be integrated as building blocks for unconventional optical components with exotic electromagnetic properties over a wide frequency range, the researchers say. Optical devices currently rely on a single frequency to transmit light.

Drs. Jie Gao (foreground) and Xiaodong Yang, both assistant professors of mechanical engineering at Missouri S&T, have designed “meta-atoms” capable of squeezing light and causing it to travel infinitely fast. Images courtesy of Missouri S&T.
The investigators created mathematical models of the meta-atom, a material 100 nm wide and 25 nm tall that combines gold and silicon oxide in a stairstep design. In simulations, 10 of the meta-atoms were stacked, and light was shone through them at various frequencies. They discovered that when light encountered the material in a range between 540 and 590 terahertz, it “stretched” into a nearly straight line and achieved an “effective permittivity” known as epsilon-near-zero.

Effective permittivity refers to the ratio of light’s speed through air to its speed as it passes through a material. When light travels through glass, for instance, its effective permittivity is 2.25. Through air or the vacuum of outer space, the ratio is one. That ratio is what is typically referred to as the speed of light.

As light passes through the engineered meta-atoms, however, its effective permittivity reaches a near-zero ratio, meaning that light actually travels faster than the speed of light.

The meta-atoms also stretch the light, while other materials, such as glass, typically compress optical waves, causing diffraction.

This stretching phenomenon means that “waves of light could tunnel through very small holes,” said Dr. Xiaodong Yang, an assistant professor of mechanical engineering. “It is like squeezing an elephant through an ultrasmall channel.”

The cross section of a 100-nm-long “meta-atom” of gold and silicon oxide. Researchers at Missouri University of Science and Technology say the meta-atom is capable of straightening and speeding up lightwaves.
The wavelength of light encountering a single meta-atom is 500 nm from peak to peak, or five times the length of Yang and Dr. Jie Gao’s theoretical stairstep meta-atoms. Although they have yet to fabricate actual meta-atoms, the investigators say their research shows that the materials could be built and used for optical communications, image processing, energy redirecting and other emerging fields, such as adaptive optics.

“The design is practical and realistic, with the potential to fabricate actual meta-atoms,” Gao said.

Previously, they built thin-film wafers from 13 stacked meta-atoms uniform in composition.

Last year, Albert Polman at the FOM Institute for Atomic and Molecular Physics in Amsterdam and Nader Engheta, an electrical engineer at the University of Pennsylvania, developed a tiny waveguide device in which lightwaves of a single wavelength also achieved epsilon-near-zero. The Missouri S&T researchers’ work, however, is the first to demonstrate epsilon-near-zero in a broadband of 50 THz.

“With this research, we filled the gap from the theoretical to the practical,” Yang said.